Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Patent
1996-08-26
1998-08-25
Langel, Wayne
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
427250, 427536, 428408, 502325, 502326, 502337, 502339, B01J 2340
Patent
active
057981485
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/GB95/00620 filed Mar. 21, 1995.
This invention relates to the production of a composite body comprising porous metal attached to a substrate.
There are applications for such composite bodies in the electrochemical field such as for gas sensors and fuel cells, as well as in the general field of catalysis of chemical reactions and catalytically acting surfaces generally, where high surface areas are required. The substrate is therefore preferably of high surface area, for example it is porous (preferably microporous), and is advantageously of an inert material such as a polymeric material such as a fluoropolymer. Microporous fluoropolymers are well known as chemically, thermally and biologically stable materials used in various phase separation situations, and are advantageously made, for the purposes of the invention, of microporous PTFE (polytetrafluoroethylene) membrane products as disclosed in European Patent 247771, which are useful in a range of applications, including metallised examples produced by for example sputter coating, electrolysis or spin coating. (In spin coating, typically, colloidal catalyst particles are impacted upon a PTFE membrane substrate by the hydrodynamic/centrifugal action of a spinning rotor immersed in the colloid.)
Although many porous Gp VIII metal composite bodies are possible according to the invention, certain materials are of especial interest to workers in the fuel cell, gas sensor and air battery fields, for example the platinum group metals including platinum, palladium and nickel.
In the electrochemical field, it is known that interlocking structures of catalyst and PTFE can be made by simple admixture of dispersions of the materials (e.g. GB Patent 1556452), but the proportion of expensive catalyst to lower-cost PTFE is very high, e.g. 10:3 by mass.
This invention seeks to increase the metal surface area of a given composite body, for example to improve the catalytic activity per unit volume of the composite body or per unit mass of catalyst metal.
U.S. Pat. No. 3715238 teaches the depolarisation of a fuel cell catalyst (e.g. Ru+Pt+some PTFE) by voltage-sweep-treating it in a selected electrolyte at 1 cycle per 1/220 minutes, whereby, gradually, catalytic metal is deposited at new surfaces so the catalytic metal surface area is gradually increasing. This is too slow as a production method, not to mention the need to rinse and dry or otherwise remove all traces of electrolyte, for some catalytic purposes. Again, the proportion of expensive metal to lower-cost PTFE is very high, nearly 10.sup.4 :1.
U.S. Pat. No. 4540476 teaches the production of a nickel electrode from a porous nickel plaque by applying an alternating potential, such that the nickel dissolves on the oxidising part of the cycle and, on the reducing part of the cycle, oxidised nickel is precipitated as hydroxide. This is performed on nickel sintered on a wire mesh support, thus foregoing the economy, porosity and high surface area per unit volume which binding as a composite with PTFE would have allowed, and also suffers from the above-noted disadvantages of a wet electrolytic process.
According to the present invention, therefore, a porous Gp VIII metal composite body is made by metallising a porous (e.g. ceramic or polymeric) substrate, preferably in the gas phase, (the ratio metal:substrate being preferably less than 1:1, preferably less than 100:1, optionally <10.sup.4 :1), oxidising the metallisation and optionally or partly or wholly reducing the metallisation, characterised in that the oxidation is performed in the gas phase by oxidative plasma and the reduction is performed in the gas phase by reducing (e.g. ammonia, hydrazine or hydrogen) plasma. The oxidative plasma may be oxygen or other anion (uni- or multi-atomic e.g. bromine) which forms a volatile product with hydrogen. In either case, the gas in question may be present at a pressure of from 0.05 to 1 Torr. Plasma treatment may be performed at a power of from 1 to 30 (pre
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Badyal Jas Pal Singh
Thomas Thomas Ronald
Boozer Tanaga Anne
British Technology Group Limited
Langel Wayne
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